bevy_reflect: Fix TypePath string concatenation (#18609)

# Objective

Fixes #18606

When a type implements `Add` for `String`, the compiler can get confused
when attempting to add a `&String` to a `String`.

Unfortunately, this seems to be [expected
behavior](https://github.com/rust-lang/rust/issues/77143#issuecomment-698369286)
which causes problems for generic types since the current `TypePath`
derive generates code that appends strings in this manner.

## Solution

Explicitly use the `Add<&str>` implementation in the `TypePath` derive
macro.

## Testing

You can test locally by running:

```
cargo check -p bevy_reflect --tests
```

crates/bevy_reflect/derive/src/string_expr.rs

@@ -80,7 +80,7 @@ impl StringExpr {
         let owned = self.into_owned();
         let borrowed = other.into_borrowed();
         Self::Owned(quote! {
-            #owned + #borrowed
+            ::core::ops::Add::<&str>::add(#owned, #borrowed)
         })
     }
 }

crates/bevy_reflect/src/lib.rs

@@ -988,6 +988,41 @@ mod tests {
         assert_eq!(values, vec![1]);
     }
 
+    /// This test ensures that we are able to reflect generic types with one or more type parameters.
+    ///
+    /// When there is an `Add` implementation for `String`, the compiler isn't able to infer the correct
+    /// type to deref to.
+    /// If we don't append the strings in the `TypePath` derive correctly (i.e. explicitly specifying the type),
+    /// we'll get a compilation error saying that "`&String` cannot be added to `String`".
+    ///
+    /// So this test just ensures that we do do that correctly.
+    ///
+    /// This problem is a known issue and is unexpectedly expected behavior:
+    /// - <https://github.com/rust-lang/rust/issues/77143>
+    /// - <https://github.com/bodil/smartstring/issues/7>
+    /// - <https://github.com/pola-rs/polars/issues/14666>
+    #[test]
+    fn should_reflect_generic() {
+        struct FakeString {}
+
+        // This implementation confuses the compiler when trying to add a `&String` to a `String`
+        impl core::ops::Add<FakeString> for String {
+            type Output = Self;
+            fn add(self, _rhs: FakeString) -> Self::Output {
+                unreachable!()
+            }
+        }
+
+        #[derive(Reflect)]
+        struct Foo<A>(A);
+
+        #[derive(Reflect)]
+        struct Bar<A, B>(A, B);
+
+        #[derive(Reflect)]
+        struct Baz<A, B, C>(A, B, C);
+    }
+
     #[test]
     fn should_reflect_clone() {
         // Struct